Inheritance Inheritance Many objects have a hierarchical relationship Examples zoo car vehicle card game airline reservation system Inheritance allows software design to take advantage of relationships supporting reuse Supports the IS A relationship what s the HAS A relationship Terminology Base class Parent class Superclass defines generic functionality Derived class Child class Subclass extends or specializes base class Syntax public class Student extends Person public class Derived extends Base public class Person void print public class Student extends Person void print Subclasses Inherit members of parent May implement new members May override members of parent Person name Person String print getName Student major Student String String print changeMajor String Method Invocation Person print getName Student print changeMajor Person p new Person Bob Student s new Student Alice CS s getName p print s print p changeMajor Econ s changeMajor Econ Method Invocation Person print getName Student print changeMajor Person p new Person Bob Student s new Student Alice CS s getName Person getName p print Person print s print Student print p changeMajor Econ ERROR s changeMajor Econ Student changeMajor Subclasses Person name Person String print getName Student major Student String String print changeMajor String Protected private members of parent not accessible to child class protected members accessible only to derived classes examples public class Person will be accessible in Student protected String name Partial Overriding in Person class void print in Student class void print super print super methodName More on super public Person String name this name name public Student String name String major super name super params this major major Exercises 1 Implement and test the Person and Student classes 1 What happens when you try to invoke changeMajor on a Person object Shadowing Variables Variable same name declared in base class and derived class Generally a bad idea Example Student declares a String variable name final Classes and methods can be defined with final modifier final public class Student final public void print final classes cannot be extended final methods cannot be overridden abstract abstract classes cannot be instantiated Declare abstract class using abstract keyword public abstract class Person Method can also be abstract public abstract void print A class with an abstract method must be declared abstract Polymorphism Many forms A variable is polymorphic if it can refer to different types of objects at different times Person p new Person Bob p new Student Sally History Example Student persondb Staff Faculty Person persondb new Person 10 persondb 0 new Student Sally History persondb 1 new Staff persondb 9 new Faculty Dynamic Binding Determine which method is called based on object contents class Person void print class Student extends Person void print Dynamic Binding Person p new Person Student s new Student p print calls Person print p s OKAY p print calls Student print p changeMajor ERROR Casting Person p p new Student Student s Student p if p instanceof Student s Student p If cast is not successful runtime error ClassCastException instanceof operator used to determine type Example Class1 1 f1 2 f2 Class1 c1 new Class2 Class2 c2 new Class2 c1 f3 c1 f2 c2 Class2 c1 c2 f3 Class2 extends Class1 3 f2 4 f3 Exercises 1 Implement the Staff and Faculty classes discussed 2 Create a PersonDB class that contains an array of Person objects Implement an addPerson method that adds a new Person object to the array and a printStudents method that prints ONLY the Student objects stored in the array Object base class All classes derive from Object Defines several methods that can be overridden String toString boolean equals Object Object clone
View Full Document
Unlocking...